Can Exchange be configured to populate a text/plain part of a meeting invite?

Yes, you can do this, but the first way I am suggesting may be a crude hack.

Add transport rules that parse the meeting summary and add it to a disclaimer (temporarily), then use this same disclaimer as an addition to each message. As Exchange will add the disclaimer with a matching format, you can output it in an HTML and text friendly manner simultaneously via use of or other similar tags. HTML is not unreadable either.

An alternative way may be to create a mailbox that receives ALL meeting invites and then replies to a specific number of clients on the to/cc/etc with a text version of the same if they match a list of text-preferential users.